Buzz in f/x loop

[MintBerryCrunch]

What could it be?

Tried all individual pedals and none make it better or worse.

Changed cables and still there.

Unplug the loop and it goes away...

[Tle4]

If it is tube driven reverb, it could be a tube.... or maybe try cleaning the loop jacks with some contact cleaner and work a plug in and out

[MintBerryCrunch]

The f/x loop is tube driven... literally peaceful without the loop engaged... time to go shopping...

[MintBerryCrunch]

new 12At7, same old hum... possibly slightly better.

I'll get some contact cleaner as suggested before I go ape**** about this

Would any other tube be even quieter, like an ECC82 maybe?

[bunny 7]

ECC 82 (12au7) is a lower gain than a 12at7 so it would be less noisy to some extent in most circumstances. The transconductance is lower than 12at7, but it's designed as a "driver" tube like a 12at7 is as far as function.
You didn't mention what amp this is in so if it's just an effects loop I'd still give it some contact cleaner and run a cable thru the jacks a few times just to clean any oxidation that might be there.
Also, check the recovery tube and make sure it's seated firmly.